1. Field of the Invention
The present invention generally relates to magnetic disk evaluation apparatuses, and more particularly, to a magnetic disk evaluation apparatus for evaluating a reliability of a magnetic disk installed in a magnetic disk device utilized as an external storage of a computer and the like.
2. Description of the Related Art
Recently a magnetic disk device has a high recording density and is miniaturized. Hence, a flying-height, defined as a distance between a surface of a magnetic disk and a magnetic head which flies from the surface of the magnetic disk, becomes lower. In order to make the flying-height low, it is necessary to minimize the surface roughness of the surface of the magnetic disk.
Meanwhile, a contact start stop (CSS) method has been mostly utilized for the magnetic device up to now. In the CSS method, when the magnetic disk stops rotating, a surface of the magnetic head comes in contact with the surface of the magnetic disk. When the magnetic device rotates, the magnetic head flies from the surface of the magnetic disk because of an air flow occurring on the basis of a rotation of the magnetic disk. If the surface roughness of the magnetic disk is low, a contacting area between a contacting surface of the magnetic head and the surface of the magnetic disk when the magnetic disk does not rotate is large. Therefore, when the magnetic disk starts rotating, an obstacle may occur between the magnetic head and the magnetic disk, and thereby it may be difficult for the magnetic head to leave the magnetic disk. In order to solve the above-mentioned problem, a texture process for the surface of the magnetic disk is applied, and thereby a minute roughness is artificially provided on the surface of the magnetic disk.
Therefore, there are two kinds of demands with respect to the magnetic disk. One is minimizing the surface roughness of the surface of the magnetic disk, in order to minimize the flying-height of the magnetic head. The other is providing the minute roughness on the surface of the magnetic disk, in order to prevent a generation of the above-mentioned obstacle between the magnetic head and the magnetic disk.
However, one of the above-mentioned demands conflicts with the other. Accordingly, for a design of a magnetic disk device, it is necessary to meet both demands under well-balanced consideration. Because of this, a magnetic disk evaluation apparatus for evaluating a guaranteed flying height of the magnetic disk is needed for designing the magnetic disk device.
Next, a method of an evaluation of the guaranteed flying height will be described. A head for an evaluation of a glide height (hereinafter a “glide height evaluation head”) can fly above the surface of the magnetic disk with a slightly lower flying-height from the surface of the magnetic disk than the flying-height of the magnetic head of the disk device product.
The glide height evaluation head also includes a contact detecting sensor such as a piezo-element for detecting a contact between the glide height evaluation head and the magnetic disk.
In the evaluation of the guaranteed flying height, when the flying-height of the glide height evaluation head is lower than a threshold value, the glide height evaluation head comes in contact with a part of the a convex part on the surface of the magnetic disk. If the flying-height of the glide height evaluation head is higher than a threshold value, the glide height evaluation head does not come in contact with the part of the convex part on the surface of the magnetic disk. Here, such threshold value, namely a lower limit value of the flying-height of the glide height evaluation head in a state where the glide height evaluation head does not come in contact with the magnetic disk, is called a “glide height”. That is, the glide height is defined as a guaranteed flying height of the magnetic disk.
Generally a magnetic disk device is set up in a state where the magnetic head flies with a flying-height being slightly higher than the glide height. Hence, if the glide height of the magnetic disk device cannot be lower than a designated flying-height of the magnetic head, the magnetic disk device loses a reliability thereof.
Since a recent magnetic disk device has a high recording density, the glide height becomes lower and the glide height evaluation head has been required to have a higher stability of flying. Furthermore, it is necessary to evaluate the magnetic disk more accurately on the basis of a detected signal which is output when the glide height head comes in contact with the magnetic disk.
However, the above-mentioned magnetic disk evaluation apparatus has a plural kinds of problems as follows.
A first problem will be described. Recently the flying-height of the magnetic head has become lower. Since the glide height evaluation head is required to have the lower flying-height than such the flying-height, a serious problem occurs if the glide height evaluation head loses a stability of the flying.
Thus, once a problem of an un-stable flying of the glide height evaluation head occurs, it may be difficult to evaluate the magnetic disk accurately. The reason why such unstable flying occurs can be presumed as follows, in proportion as a case of the magnetic head.
(1) First of all, a lubricant, a contamination such as a minute and dirty molecule, or the like, moves from the surface of the magnetic disk and absorbs onto a flying surface of the glide height evaluation head which flies with a low flying-height.
(2) Next, if an amount of the absorption onto the flying surface increases to a designated amount or more, the flying property of the glide height evaluation head is changed, and thereby the flying-height of the glide height evaluation head is changed. The flying-height is declined in many cases. In a worst case, the glide height evaluation head may fall down onto the surface of the magnetic disk device, and thereby the glide height evaluation head may slide on the surface of the magnetic disk.
(3) After an absorbed material onto the flying surface of the glide height evaluation head is removed because of sliding, the glide height evaluation head may have the designated flying-height again, and thereby the glide height evaluation head flies stably again.
(4) The processes of the above mentioned (1) to (3) are repeated irregularly. Because of this, it may be difficult to keep a stable flying for a long period of time.
According to the above-mentioned presumption, the absorption of the lubricant, contamination, or the like onto the flying surface of the glide height evaluation head may be a main reason why the unstable flying occurs. However, if the unstable flying occurs actually and the glide height evaluation head is changed, the situation of the unstable flying is also changed. Hence, it is clarified that the above-mentioned situation of the unstable flying is not as simple as it is understood on the basis of the above-mentioned presumption.
Next, a second problem will be described. It is a main object of the evaluation by the conventional magnetic disk evaluation apparatus to decide whether or not the surface of the magnet disk has an unusual convex part. For the purpose, the glide height evaluation head flies with a designated flying-height namely a glide height and runs above the surface of the magnetic disk which rotates, and thereby makes a decision of whether or not the convex part comes in contact with the glide height evaluation head exists on the magnetic disk. The contact detecting sensor such as the piezo-element for detecting the contact between the glide height evaluation head and the convex part of the magnetic disk is provided in the glide height evaluation head.
However, as described above, it is necessary to decrease the flying-height of magnetic head. Hence, not only the contact between the convex part of the surface of the magnetic disk and the magnetic head but also a partial convex part of the lubricant generated by a lack of uniformity of applying the lubricant on the surface of the magnetic disk and the magnetic head, may be apt to occur as another problem.
Therefore, it is necessary to detect the lubricant having a convex shape and formed due to the lack of uniformity of applying the lubricant, by the magnetic disc evaluation apparatus. However, the conventional magnetic disc evaluation apparatus has a structure for detecting a contact with the convex part (solid) on the surface of the magnetic disk. Accordingly, up to now, there is neither an idea to detect a convex part of the lubricant (liquid) nor a suggestion about an effective evaluation thereof.
In the conventional magnetic disk evaluation, in order to detect the contact between the glide height evaluation head and the convex part on the magnetic disk, a vibration frequency of the glide height evaluation head which is an object of the operation is set as for example approximately 200 to 800 kHz. The glide height evaluation head coming in contact with the solid convex part has a tendency in that the glide height evaluation head vibrates with a high frequency of 200 kHz or more for example.
Accordingly, it is judged that the glide height evaluation head comes in contact with the solid convex part, if a signal ingredient having a higher frequency than 200 kHz in the detected signals detected by a contact detect sensor exceeds a designated value. Furthermore, a vibration based on a lower frequency than 200 kHz is eliminated from an object to be detected because it can be presumed that the vibration based on the lower frequency than 200 kHz may be caused by an influence of a vibration of an air film near the glide height evaluation head.